Hot melt adhesives, 100% solids materials, which are applied to a substrate when molten and cooled to harden the adhesive layer, are widely used for industrial applications.
Current hot melt pressure sensitive adhesive technology utilizes either styrene-butadiene-styrene, "SBS" or styrene-isoprene-styrene, "SIS" elastomers in the adhesive composition. SBS and SIS are not compatible with each other and therefore are not usually used in the same adhesive composition.
U.S. Pat. No. 5,290,842 discloses pressure sensitive adhesives comprising both SIS and SBS, however this composition is not compatible and has inferior performance properties.
Both SIS and SBS have been used separately to formulate hot melt pressure sensitive adhesives. Unfortunately, SIS and SBS are naturally incompatible, and thus a SIS/SBS blended pressure sensitive adhesive usually does not have good adhesion properties.
It has been found, in accordance with the present invention, that blends of SIS and SBS can be made compatible by utilizing their common styrene end-blocks to enhance the compatibility of their dissimilar mid-blocks. Two types of resins can be utilized to enhance the compatibility of SIS and SBS. One is end-block compatible resin and the other is both mid- and end-block compatible resin. The end-block compatible resin can enlarge and reinforce the end-block domains to force the mixing of the two mid-block, while the mid/end-block compatible resin can function as a good solvent for both the polymers to solubilize the two mid-blocks together. As used herein, the "midblock" of the polymer refers to polymeric blocks which are substantially aliphatic. "Endblock" of the polymer refers to polymeric blocks which are substantially aromatic.
The present invention provides a hot melt pressure sensitive adhesive based on a compatibilized blend of SIS and SBS elastomers which displays excellent adhesion properties compared to SIS, SBS or incompatible blends. These adhesives will also have improved thermal stability and aging properties compared to SIS based or SBS based formulations. SIS undergoes thermal degradation and its melt viscosity decreases very rapidly during hot melt processing, while SBS experiences thermal crosslinking and its melt viscosity increases quickly. As a result, blends of SIS and SBS, if compatibilized, will show less change in the melt viscosity and the adhesion properties after aging.